1. Field of the Invention
The present invention relates to a catalyst and to a process for preparing low-viscosity and color-reduced polyisocyanates containing isocyanurate groups.
2. Description of the Background
For high-quality one- and two-component polyurethane coating materials possessing high light and weathering stability, polyisocyanate mixtures containing isocyanurate groups and uretdione groups are employed, in particular, as the isocyanate component.
For the preparation of polyisocyanates containing isocyanurate groups and uretdione groups, which are suitable as raw materials for polyurethane coating formulations, a variety of processes are known. These processes differ, generally speaking, in the selection of the trimerization catalysts or else in the selection of the organic isocyanates to be used in the oligomerization reaction (cf., e.g., GB-B 1391066, EP 82 987, DE 390 2078, DE 339 396, EP 224 165).
Isocyanates suitable for trimerization, examples being aromatic, cycloaliphatic and aliphatic polyisocyanates with an isocyanate functionality of two or more, may be prepared by various kinds of processes (Annalen der Chemie 562 (1949), pages 75 ff.). The processes which have proven to be particularly suitable in industry include preparation of an isocyanate by phosgenation of organic polyamines to the corresponding polycarbamoyl chlorides followed by thermal dehydrochlorination of the chlorides into organic polyisocyanates and hydrogen chloride.
Alternatively, organic polyisocyanates may be prepared without the use of phosgene, i.e., by phosgene-free processes. According to EP 0 126 299 (U.S. Pat. No. 4,596,678), EP 130 126 300 (U.S. Pat. No. 4,596,679) and EP 0 355 443 (U.S. Pat. No. 5,087,739), for example, (cyclo)aliphatic diisocyanates, such as 1,6-hexamethylene diisocyanate (HDI) and/or isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene radical and 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate (IPDI)), may be prepared by reacting the parent (cyclo)aliphatic diamines with urea and alcohols to give (cyclo)aliphatic biscarbamic esters and thermally eliminating alcohol from these esters thereby yielding the corresponding diisocyanates and alcohols. The synthesis takes place continuously in a circulation process and in the presence, if desired, of N-unsubstituted carbamic esters, dialkyl carbonates, and other by-products returned from the reaction process.
Examples of catalysts which may be used for the trimerization of isocyanates to give the desired polyisocyanates containing isocyanurate and uretdione groups are tertiary amines, phosphines, alkali metal phenoxides, aminosilanes, quaternary ammonium hydroxides, and quaternary ammonium carbonates. Other highly suitable oligomerization catalysts are hydroxides, halides or carboxylates of hydroxyalkylammonium ions (cf., e.g., EP 351 873, U.S. Pat. No. 5,290,902), alkali metal salts, and also tin salts, zinc salts and/or lead salts of alkylcarboxylic acids. Depending on the catalyst, it is also possible to use various cocatalysts such as, for example, OH-functionalized compounds or Mannich bases comprising secondary amines and aldehydes and/or ketones.
For the oligomerization, the (cyclo)aliphatic diisocyanates are reacted in the presence of a catalyst, with or without the use of solvents and/or auxiliaries, until the desired conversion is attained. Partial trimerization is one of the terms used in this context, since the target conversion is generally well-below 100%. Subsequently, the reaction is terminated by deactivation of the catalyst and the excess monomeric diisocyanate is usually separated, generally by flash distillation or thin-film distillation. Deactivation is conducted by thermal treatment or by adding a catalyst inhibitor such as, for example, p-toluenesulfonic acid or bis(2-ethylhexyl)phosphate. Particularly advantageous, in the context of the trimerization of isocyanates on the industrial scale, is the use of quaternary hydroxyalkylammonium carboxylates as oligomerization catalysts. These catalysts of the choline type are thermally labile. It is unnecessary to terminate the trimerization on reaching the desired conversion by adding catalyst inhibitors which have the potential to reduce the quality. Instead, the controlled thermal deactivation permits optimum process control. This thermal stability is also advantageous from the standpoint of process safety. Uncontrolled “runaway” of the reaction is impossible, provided the amount of catalyst metered in remains below a multiple of the usual amount.
Depending on the type of catalyst used and the reaction temperature, the resulting polyisocyanates have different fractions of isocyanurate groups and uretdione groups, respectively. The products are usually clear, although products with a more or less strong yellow coloration may also be obtained depending on the type of catalyst, quality of diisocyanate, temperature of reaction, and reaction regime. For the preparation of high quality polyurethane coating materials, however, products having a very low color number are required.
In the light of ongoing legislative concerns to monitor and restrict the emission of volatile organic compounds (known as VOCs), coatings manufacturers are continually under pressure to reduce the solvent content of their formulations. Complying with the strict statutory requirements is no trivial task. Using the solvent, coating formulations are adjusted to a viscosity which ensures optimum processing properties and sprayability. If the solvent content is reduced, the viscosity rises automatically and the processing parameters of the formulation are significantly impaired. The problem can, however, be countered by using binder components of especially low viscosity to prepare the low-solvent systems, known as low VOC coating materials. On the part of the manufacturers of PU formulations, accordingly, there is an urgent need for polyisocyanates which contain isocyanurate groups and which are of good color quality and at the same time feature low viscosity. The latter is true in particular of polyisocyanates based on IPDI (isophorone diisocyanate) and NBDI (2,5(2,6)-bis(isocyanatomethyl)bicyclo[2.2.1]heptane), which have a particularly high viscosity in the form in which they have been freed from monomer.
The trimerization of diisocyanates produces not only the ideal trimer (monoisocyanurate), but also the pentamer, the heptamer, and higher oligomers. The viscosity of the demonomerized polyisocyanate increases as the oligomer content rises. In principle, the oligomer content of a polyisocyanate containing isocyanurate groups is in inverse proportion to the degree of conversion; consequently, it may be controlled via the conversion of the partial trimerization. Where a suitably low conversion is directed, the viscosity of the resultant product is also low. This procedure, however, is very uneconomic. U.S. Pat. No. 5,691,440 describes trimerization catalysts which, independently of the process and with a comparable conversion, provide a lower oligomer content and hence a lower-viscosity product than the prior art catalysts (column 2, line 67 to column 3, line 3). With the aid of these catalysts, which comprise a limited selection of specific tetraalkylammonium carboxylates (claim 1), even the demanding IPDI may be trimerized to a low-oligomer product while maintaining high and hence economic conversions (column 3, line 3-37).
The catalysts of U.S. Pat. No. 5,691,440 therefore enable economic access to low-viscosity polyisocyanates containing isocyanurate groups, even when using demanding diisocyanates such as IPDI as raw material. A disadvantage, however, is that the products have an unwanted yellow coloration. Their color quality is deserving of optimization, because, as already mentioned, products having an extremely low color number are required for the preparation of high-quality polyurethane coating materials. A need continues to exist for a catalyst system which provides a low-viscosity polyisocyanate which contains isocyanurate groups and is of improved color quality.